Insulation of the floor slab

Q: Which insulation materials are suitable under the floor slab?

As a rule, the following are used:cellular glass: non-combustible, pressure-resistant, vapor-tight, durable and fully recyclableXPS: good insulation performance, but petroleum-based, flammable and difficult to recycleEPS: less pressure-resistant, sensitive to moisture – usually unsuitable for load-bearing floor slabsCellular glass is the only mineral insulation material with an optimum combination of environmental friendliness, technical safety and durability – ideal for sustainable new buildings.

Insulating the floor slab saves heating costs, reduces the risk of mold and creates future-proof living comfort.

Structure, function and variants of floor slab insulation

Considerable amounts of heat are also lost through floor slabs. In order to reliably prevent heat loss, moisture ingress and the risk of mold, professional thermal insulation is mandatory. In new buildings, insulation in accordance with DIN 4108 is now predominantly installed under the floor slab – in existing buildings, it is often installed above the floor slab.

Insulation under the floor slab(perimeter insulation): This design is now the standard in new buildings and ensures a permanently dry and energy-efficient floor structure, both with GLAPOR cellular glass boards and the GLAPOR RDS – Combi perimeter insulation system with cellular glass gravel.
Insulation above the floor slab(screed insulation): Primarily used in renovation projects when the floor slab cannot be opened. The insulation material made of cellular glass boards in accordance with EN 13167 lies above the existing construction and is protected by screed or dry screed.

The GEG-U value for floor slabs in contact with the ground is a maximum of 0.30 W/(m²-K). To achieve this, insulation thicknesses of approx. 10-20 cm are required, depending on the insulation material and thermal conductivity (λ).

Innovation: Insulating the floor slab has never been so easy!

GLAPOR RDS combination perimeter system

Plastic-free, waterproof, pressure-resistant. cellular glass boards as certified formwork. Cellular glass gravel as load-bearing thermal insulation. In combination, the healthy basis for every house.

UMB1: Urban mining base plate

The world’s first 100% circular floor slab without concrete. Quickly laid – up to 100 m² per day. Insulates reliably, nominal pressure resistance up to 1600 kPa and saves up to 95 % CO₂.

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Insulation under the floor slab

In new buildings, the insulation layer is installed directly under the load-bearing floor slab. In areas with pressing water, this can be realized with GLAPOR cellular glass boards – in areas with non-pressing water, the GLAPOR RDS combination perimeter insulation system is suitable

Exemplary structure with insulation under the floor slab:

Subgrade or lean concrete
Pressure-resistant insulation (e.g. SG 800 P cellular glass gravel, load-bearing)
PE release film against cement milk
Concrete slab
Base insulation against thermal bridges

The materials used must be highly pressure-resistant, moisture-resistant and rot-proof – properties that cellular glass ideally fulfills.

Insulation on the floor slab

In old buildings, it is often only possible to insulate the existing floor slab with cellular glass boards, for example. A vapor barrier is applied, then the insulation is laid and covered with screed.

Typical structure:

Clean and level floor slab, create waterproofing
Cemwood 1000 leveling fill
GLAPOR cellular glass boards
Separating layer / cover
Screed or dry screed
Flooring (parquet, tiles, carpet etc.)

Risks:

Room height is noticeably reduced
Frames, doors, thresholds must be adapted
Thermal bridges possible on the outer wall

Better alternative (if possible): Excavate the floor, install cellular glass gravel as a capillary-breaking insulating layer and apply a lightweight screed on top. More information on ecological floor renovation.

Comparison of floor slab insulation materials

Criterion	GLAPOR cellular glass boards	XPS
Raw material	✓ 100 % recycled glass	Petroleum based
Water absorption	✓ None (vapor-tight)	Possible through diffusion
Compressive strength	✓ Up to 1,600 kPa - also suitable for parking decks	Max. 700 kPa
Fire protection class	✓ A1 (non-combustible)	B1-B2 (flammable)
Emissions / leaching	✓ None - emission-free	Waste disperses during processing
Waste disposal	✓ Reusable, urban mining ready	Construction waste or hazardous waste
Lifespan	✓ Lifelong - across generations	Characteristics fluctuate with moistening

Why cellular glass is the best choice for insulating the floor slab

Whether a residential building or an industrial hall: the floor slab is the foundation for safety, energy efficiency and durability. A reliable insulating material is needed here:

Pressure-resistant & dimensionally stable: no subsidence, no deformation
Vapor-tight & rot-proof: no moisture ingress, no damage
Non-flammable: safety even in the event of fire
Plastic-free & recyclable: For DGNB, EU taxonomy and urban mining projects

Available as slabs or gravel, GLAPOR cellular glass can be adapted precisely to the structural, thermal and ecological requirements of modern construction projects.

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cellular glass 100% made in Germany

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✔ Customized project consulting

cellular glass 100% made in Germany

✔ 100,000,000+ m³ of cellular glass installed

Frequently asked questions (FAQ) about floor slab insulation

FAQ

Does insulation under the floor slab make sense?

Yes, insulation under the floor slab is particularly useful. It minimizes heat loss, increases energy efficiency and provides reliable protection against rising damp. As the insulation layer is outside the heated area, the thermal mass of the floor slab is retained – an advantage for the indoor climate and comfort.

How many cm of insulation goes under a floor slab?

The required insulation thickness depends on the desired U-value and the thermal conductivity of the insulation material used. According to the Building Energy Act (GEG), a maximum U-value of 0.30 W/(m²-K) is prescribed, which leads to minimum insulation thicknesses of 10-20 cm – depending on the material.

Why is a foil laid between the insulation and the floor slab?

A PE separating film protects the insulation layer during concreting. It prevents cement milk from penetrating the insulation, which could impair the insulating effect or damage the board structure. This is particularly critical with open cell structures such as EPS. The risk is lower with vapor-tight cellular glass – but a film is still recommended here to ensure a clean separation.

How many cm of insulation do you need on the floor slab?

The Building Energy Act, which stipulates a maximum U-value of 0.3, also applies here. Minimum insulation thicknesses of 10-20 cm – depending on the material – are therefore required on the floor slab.

Which insulation materials are suitable under the floor slab?

As a rule, the following are used:

cellular glass: non-combustible, pressure-resistant, vapor-tight, durable and fully recyclable
XPS: good insulation performance, but petroleum-based, flammable and difficult to recycle
EPS: less pressure-resistant, sensitive to moisture – usually unsuitable for load-bearing floor slabs

Cellular glass is the only mineral insulation material with an optimum combination of environmental friendliness, technical safety and durability – ideal for sustainable new buildings.

How much does a floor slab with insulation cost?

The costs for the floor slab including insulation depend on the area, insulation material and thickness as well as the implementation and vary between 100-200 €/m².